Shedding light on arctic zooplankton in the dark
Some of the smallest creatures on the planet — zooplankton — make the most widespread vertical migration of biomass on Earth. Billions of these animals move deeper into the ocean and away from the light during the day to avoid predators, and migrate up again in the dark of night to feed.
Over the last decade, scientists have pieced together the story of what happens with this migration in the Arctic, where it is dark for up to six months of the year. Recent research, including from the Norwegian University of Science and Technology (NTNU), shows that even the weak light of the Moon is enough to trigger a rapid migration to deeper and darker waters. The light from the aurora will also cause this phenomenon.
But this extreme sensitivity to light has a dark side: the light from vessels or other manmade ocean structures is enough to disturb zooplankton movement, and drive the animals into the depths. That can affect scientists' ability to accurately estimate just how many of the creatures there are.
Modified kayak detects light pollution
A team of researchers from NTNU, UiT -The Arctic University of Norway, the University of Delaware and the Scottish Association for Marine Science used a modified kayak equipped with sensors to discover this potential for light pollution. Their article about the problem appeared in a recent issue of Science Advances.
The researchers also believe the zooplankton's light sensitivity explains why no one has ever before been able to document how zooplankton respond to weak light from the sun in the dark of the polar night.
Basically, whenever a lighted research vessel sails into dark polar waters, its lights drive the zooplankton away. That means the researchers can't measure what truly happens to the tiny animals in the dark.
"These findings tell us that zooplankton populations and behavior can be under- or overestimated because these marine organisms respond to light, either by swimming away from it, or sometimes towards it," said Geir Johnsen, a marine biologist at NTNU who was involved in the study.
Johnsen says if biologists really want to know what is happening with zooplankton, they need to undertake their studies under natural, ambient light conditions.
Finding zooplankton — with no light
Members of the team had already spent several winters studying marine life during the polar night, and they suspected that the light from their research vessel might be scaring fish and zooplankton away, said Martin Ludvigsen, a professor in NTNU's Department of Marine Technology and at the university's Centre of Autonomous Marine Operations and Systems.
"We did have a suspicion that this was the case," Ludvigsen said. "And we were able to demonstrate this, and show the significance of the lights from the ship."
The research team used a specially engineered vessel called a Jetyak, which is what it sounds like: a kayak outfitted with a petrol engine that drives a water jet unit on the back of the boat so that it is powered independently.
In January 2016, the Jetyak was programmed to follow predefined transects on three different days in Kongsfjorden, a fjord on the west coast of the Svalbard archipelago. The Jetyak measured ambient light and used an acoustic instrument to detect zooplankton layers in the water column. The Jetyak was thus able to move away from the lighted research vessel under its own power, and collect data about the location of zooplankton in the water column in the dark.
Fifteen times more light
At the beginning and end of each of the three deployments when the Jetyak returned to the lighted research boat, the amount of ambient light its instruments detected increased by 15 times.
The researchers were also able to see in the acoustic data collected by the Jetyak that the layer of zooplankton near the boat was far deeper in the water column than zooplankton that were away from the lights of the research boat. This effect reached depths of up to 80 metres.
"We were sort of surprised how pronounced this avoidance behavior was," Ludvigsen said. "It was so clear and so fast. Even when we tried to reproduce this in a small boat and a headlamp, it was really easy to see in the echosounder."
An ice-free Arctic amplifies problem
Arctic species, from bowhead whales to marine birds to cod, feast on zooplankton, particularly those in the genus Calanus, which load up on fatty acids so that they can survive the Arctic winter. This storehouse of fatty acids is what makes them such a great meal. But even a little bit of light pollution may affect this delicate relationship, Johnsen said.
"Light pollution may disturb zooplankton behavior with respect to feeding, predator-prey relationships and diurnal migration, in addition to their development from juveniles to adults," he said.
Global warming has the potential to exacerbate this disturbance by reducing the amount of Arctic sea ice. A protective cover of sea ice in the winter can shield zooplankton from light pollution, but if sea ice decreases, it will be easier for artificial light to penetrate the ocean, Johnsen said.
Oil and gas exploration in the Arctic will also bring more light pollution to the region, as could a potential increase in shipping traffic from when Arctic shipping lanes are free of ice.
A big ocean made smaller by autonomous vehicles
Ludvigsen says the finding underscores the importance of robotic data collection when it comes to studying the ocean.
Autonomous craft, particularly those that incorporate some kind of artificial intelligence so they can make real-time decisions while collecting data, will allow better and more sampling, he said.
"Sampling the ocean is challenging, and in most cases we are undersampled," he said. "Vehicles that are autonomous can optimize the effort they use to increase the information we get from a mission."
Another benefit is that these vehicles have a much lower impact in the Arctic in terms of noise and light, he said.
"The ocean is so, so big," Ludvigsen said. "We can see the surface from satellites, but we need to go to deeper depths."
Reference: Ludvigsen M, Berge J, Geoffroy M, Cohen JH, De La Torre PR, Nornes SM, Singh H, Sørensen AJ, Daase M, Johnsen G (2017) Use of an Autonomous Surface Vehicle reveal new zooplankton behavioral patterns and susceptibility to light pollution during the polar night. Science Advances 4, eaap9887. DOI: 10.1126/sciadv.aap9887.